Problem: In chemistry class, the girl to boy ratio is $5$ to $7$. If there are a total of $36$ students, how many girls are there?
A ratio of $5$ girls to $7$ boys means that a set of $12$ students will have $5$ girls and $7$ boys. A class of $36$ students has $3$ sets of $12$ students. Because we know that there are $5$ girls in each set of $12$ students, the class must have $3$ groups of $5$ girls each. There is a total of $15$ girls in chemistry class.